I am also having the same problem with a Duet I purchased last November.
I just recorded with it for the first time last night. I'm using a Rode NT2 plugged into Input 1 on the Duet. The Input setting is for XLR Mic (so please don't bother mentioning the +4 or -10dB settings, thank you).
I brought the levels up to where I got a proper level for my acoustic guitar (no amp). I noticed a HUGE amount of hiss.
I also tried what ClayG said in another thread, and disconnected the mic from the input, then turned the level all the way up, and I found the source of the hiss: It had nothing to do with my room noise (which is almost non-existent), the hiss came from the Apogee.
I'm really disappointed with this. Everyone is always raving about how quiet this thing is supposed to be, but I've never heard an interface so noisy before.
Anyone who says their Duet is quiet, please perform the following test before responding: Disconnect the mic from the input; turn on phantom power, and put the level of the input all the way up, and listen with headphones. Do you hear hiss and static? I hear a lot, and this is obviously the source of the hiss on my recording.
I can record with an acceptable level of hiss with the input level set to 43 or below. Above that, the hiss is very very noticeable. Unfortunately, 43 is too low to get a proper level for a quiet acoustic instrument.
Also getting very buggy behavior with the Maestro software (version 1.7.1).
I'm very disappointed with all the problems I've had with the Apogee Duet.